The present invention relates to an electrochromic or photoelectrochromic device, particularly suitable for reversible storage and display of data and for the control of light transmission, making use of one or two electrodes made of semiconductor having high specific surface area.
In order to prepare an electrochromic device in which the changing of visible light absorption is greater than 90% (for example from 5 to 95%), and which uses molecules as electrochromic units, it is necessary for the surfaces to have densities reaching the value of xcex93 greater than 5xc3x9710xe2x88x928 mol/cm2 if the extinction coefficient of these molecules varies from xcex94xcex5=20""000 when changing oxidation state. Up to now, this requirement has been met in the following manner when preparing electrochromic devices:
the electrochromic compound is present in solution contacting the electrode. The requirement is fulfilled with a 0.25 M concentration and a layer thickness of the solution of 2 xcexcm;
the electrochromic compound is electrochemically precipitated in a thin layer on an electrode;
the electrochromic compound is polymerised or incorporated into a film of polymer or composite material on the surface.
A description is provided hereunter of a new type of electrochromic and photoelectrochromic devices, which, in order to attain the performance characteristics indicated earlier, make use of electrodes made of nanocrystalline semiconductor having very high specific surface area on the surface from which electrodes the electrochromic molecules are adsorbed. These devices are rapid, with switching times of less than 3 s for an absorption change of at least 90%, and allowing for brilliant colours to be obtained.
The invention relates to electrochemical systems comprising at least two electrodes, each of which may be transparent or opaque, and at least one of which changes colour depending on:
I. the voltage applied between the two electrodes by a current-voltage source;
II. the intensity of the light to which the system is exposed;
III. the combined influence of I and II.
Furthermore, the colour change engendered by the light may be
A. local: only the lit location changes colour;
B. global: the entire system changes colour, irrespective of the location of the lighting.
Furthermore, the colour change engendered by the external voltage may be:
a. global: the entire system changes colour;
b. local: only the electrically addressed location changes colour (structured surface of the electrode).
This results in the following applications:
I-a: reversible electrochromic systems for the control of light transmission, governed by an external voltage-current source;
I-b: electrochromic systems for the reversible data display, governed by an external voltage-current source;
III-A: reversible photoelectrochromic systems for the optical writing and reversible data storage, controlled by a light beam (writing) and by an external voltage-current source (storage and deletion);
II-B: systems (filters, glazing), of which the transmission adapts automatically to the intensity of the light received.
All these colour changes, easy to observe with the naked eye except in the case of III-A when data storage is on micrometric and sub-micrometric scale, correspond to chemical reactions which are well defined on the molecular level, namely oxidations or reductions of an electrochromic compound, usually grafted onto the whole of the surface of an electrode made of nanocrystalline semiconductor accessible to such molecules. Such an electrode is prepared by sol-gel process such as the one described in detail by Stalder and Augustynski in J. Electrochem. Soc. 1979, 126, 2007, while maintaining the relative humidity of the ambient air at a value of between 50 and 80%, without a variation of more than 5%, during the hydrolysis of the metal alcoholate of which metal the oxide is being prepared. The thickness of the nanocrystalline layer is between 0.1 and 10 xcexcm or more, leading to a roughness factor of between about 10 and 1000, for example 700, meaning that the electrode surface area which is accessible to molecules having a typical diameter of 1 nm is 10 to 1000 times the value of the projected layer surface; e.g. 700 times. The result of this is that any change in the optical properties of a layer of molecules adsorbed on the surface of the semiconductor will engender macroscopic effects amplified by the roughness factor. Accordingly, the light absorption by a monolayer of coloured molecules will be stronger by a factor equal to the roughness factor on a nanocrystalline electrode than on a flat surface.
Semiconductors which are particularly suitable for the preparation of the nanocrystalline electrodes must possess a large band gap. They may be chosen from among the oxides of the elements from Group IV of the periodic system, e.g. titanium, zirconium, or hafnium, from Group V, e.g. vanadium, niobium, or tantalum, from Group VI, e.g. chromium, molybdenum, or tungsten, or from other groups, e.g. silver, zinc, strontium, iron, or nickel. They may equally be of the perovskite type, such as SrTiO3 or CaTiO3.
In particular, the invention relates to an electrochromic or photoelectrochromic device possessing the property of changing colour under the effect of an electric voltage and/or of a variation in the intensity of a light radiation, this device comprising at least one cathode and one anode, at least one of these electrodes being constituted at least in part of a transparent or translucent substrate bearing an electrically conductive coating, and an electrolyte arranged between these electrodes, and an electric circuit connecting said cathode and anode, this device being characterised in that at least one of these electrodes carries a coating constituted of at least one nanocrystalline layer of at least one semiconductive material, having a roughness factor equal to at least 20, and a monolayer of electrically active molecules or of an electrically active polymer, said monolayer being adsorbed on the surface of this coating, and in that the device contains at least one auxiliary electrically active compound, possibly dissolved in the electrolyte, having the property of being capable of being oxidised or reduced in a reversible manner.
It is in particular possible to envisage embodiments which present one or more of the following specific features:
a. the semiconductor is a titanium-, zirconium-, hafnium-, vanadium-, niobium-, tantalum-, molybdenum-, tungsten-, zinc-, strontium-, iron-, nickel-, silver-oxide or a perovskite of the said metals;
b. the electrical circuit comprises a current-voltage source;
c. the device comprises a small auxiliary electrode in addition to the anode and the cathode;
It is in particular also possible to achieve a number of variants of the electrochromic device according to the invention, each of these variants presenting special features according to one of the following points:
1. variant in which the cathode carries an adsorbed monolayer of at least one type of electrochromophoric molecules, which molecules comprise at least one adsorbable attachment group, possibly a polymerisable or condensible group, and at least one type n electrochromophoric group of which the property is to be colourless in the oxidised state and coloured in the reduced state, the auxiliary electroactive compound being fixed at the anode in the form of an electroactive coating, the electrolytic solution between the electrodes containing at least one electrochemically inert salt in solution in a solvent;
2. variant according to point 1, in which the electrochromophoric molecules comprise, as the electrochromophoric group, N,Nxe2x80x2-dialkylbipyridinium or the diimide derivative of naphthalene-1,4,5,8-tetracarboxylic acid;
3. variant according to point 1, in which the electrochromophoric molecules comprise, as the attachment group, the carboxylate, salicylate, catecholate or phosphonate group, and, if applicable, as the polymerisable group, the vinyl or pyrrole group, or, as the condensible group, the alcohol or amine group;
4. variant according to point 1, in which the cathode and anode are transparent;
5. variant according to point 1, in which a reflective screen is placed behind the cathode and the system is therefore opaque;
6. variant according to point 5, in which the reflective screen is constituted of a microcrystalline semiconductor layer, said semiconductor layer being as indicated in point a above, deposited on the cathode face located inside the system;
7. variant according to point 6, in which the anode is a metal plate;
8. variant according to point 7, in which the said metal is zinc;
9. variant according to point 1, in which the said electroactive coating is constituted of a dense electrochemically deposited layer;
10. variant according to point 9, in which this dense layer is reduced Prussian blue (xe2x80x9cPrussian whitexe2x80x9d, poly-ferrocyanide iron(II));
11. variant according to point 9, in which this dense layer is an electroactive organic polymer;
12. variant according to point 9, in which this dense layer is a composite material comprising an electroactive material;
13. variant according to point 1, in which the electroactive coating is constituted of a nanocrystalline semiconductor layer, of which the roughness factor is greater than 20, on the surface of which is adsorbed a monolayer of electroactive molecules or an electroactive polymer;
14. variant according to point 13, in which the electroactive molecules are electrochromophoric molecule comprising an adsorbable attachment group, possibly a polymerisable or condensible group, and a type p electrochromophoric group of which the property is to be colourless in the reduced state and coloured in the oxidised state;
15. variant according to point 14, in which the said electrochromophoric molecules comprise attachment groups and polymerisable group according to point 3;
16. variant according to point 7, in which the electrolytic solution likewise contains a metal salt of which metal the anode is constituted;
17. variant according to point 1, in which the said solvent is an electrochemically inert liquid salt;
18. variant according to point 17, in which the said liquid salt is of 1-ethyl-3 methylimidazolium- or 1-propyl-2,3-dimethylimidazolium trifluoromethane-sulfonate or -bis(trifluoromethylsulfonyl)amide;
19. variant according to point 1, in which the said solvent is acetonitrile, butyronitrile, glutaronitrile, methoxypropionitrile, dimethlysulfoxide, sulfolane, dimethylformamide, dimethylacetamide, N-methyl oxazolidinone, dimethyl-tetrahydro-pyrimidinone (DMPU);
20. variant according to point 1, in which the said electrochemically inert salt or salts are selected from among tetraalkylammonium-, 1,3-dialkylimidazolium- or lithium hexafluorophosphate, -trifluoromethanesulfonate, -bis(trifluoro-methylsulfonyl)amide, or -perchlorate.
Likewise, it is possible to achieve in particular a number of variants of the photoelectrochromic device according to the invention, each of these variants presenting special features according to one of the following points:
21. variant in which the coloration of the device adapts automatically to the intensity of the light;
22. variant according to point 21, in which the anode carries a nanocrystalline semiconductor layer, said semiconductor being in accordance with point a above, of which the roughness factor is greater than 20, on the surface of which semiconductor a monolayer of a sensitising agent is adsorbed, said sensitising agent comprising a chromophoric group, an adsorbable attachment group, and possibly a polymerisable or condensible group;
23. variant according to point 21, in which the anode carries a nanocrystalline semiconductor layer, said semiconductor being in accordance with point a above, of which the roughness factor is greater than 20, on the surface of which semiconductor a monolayer of electrochromophoric molecules is adsorbed, said molecules comprising an adsorbable attachment group, an type p electrochromophoric group of which the property is to be colourless in the reduced state and coloured in the oxidised state, and possibly a polymerisable or condensible group;
24. variant according to point 21, in which the anode bears a nanocrystalline semiconductor layer, said semiconductor being in accordance with point a above, of which the roughness factor is greater than 20, lacking in adsorbed molecules;
25. variant according to point 22, in which the said sensitising agent comprises a type p electrochromophoric group linked to the chromophore of which the property is to be colourless in the reduced state and coloured in the oxidised state;
26. variant according to point 22, in which the said sensitising agent and the said electrochromophoric molecules are co-adsorbed on the anode, in the proportions of 1 to 1, of 1 to 2, or of 1 to 5 or more;
27. variant according to point 21, in which the cathode carries a nanocrystalline semiconductor layer, said semiconductor being in accordance with point a above, of which the roughness factor is greater than 20, on the surface of which semiconductor is adsorbed a monolayer of at least one type of electrochromophoric molecules, which comprise at least one adsorbable attachment group, at least one type n electrochromophoric group of which the property is to be colourless in the oxidised state and coloured in the reduced state, and possibly a polymerisable or condensible group;
28. variant according to point 21, in which the cathode does not carry any nanocrystalline semiconductor layer;
29. variant according to point 21, in which the said electroactive auxiliary compound is an electrochemically active salt, capable of transporting electrons between cathode and anode, dissolved in the said solution;
30. variant according to point 21, in which the said electroactive auxiliary compound is a type p electrochromophoric group linked to the chromophore according to point 25, and in which the said solution only contains electrochemically inactive salts;
31. variant according to point 29, in which the said electrochemically active salt is an type p or type n electrochromophore in solution;
32. variant according to points 22, 23, and 27, in which the said electrochromophoric molecules comprise attachment groups and polymerisable groups according to point 3;
33. variant according to point 21, adapting favourably to visible light, in which the cathode is made in accordance with point 27 and in which the anode is made in accordance with point 22;
34. variant according to point 33, adapting favourably to visible light, made in accordance with point 29;
35. variant according to point 33, adapting favourably to visible light, made in accordance with point 31, in which the said electrochromophore in solution is of type p;
36. variant according to point 21, adapting favourably to visible light, in which the cathode is made in accordance with point 27, and in which the said solution is made in accordance with point 29;
37. variant according to point 36, adapting favourably to visible light, in which the anode is made in accordance with point 25;
38. variant according to point 36, adapting favourably to visible light, in which the anode is made in accordance with point 26;
39. variant according to point 21, adapting favourably to ultra-violet, in which the cathode is made in accordance with point 27, in which the anode is made in accordance with point 23, and in which the said solution is made in accordance with point 29;
40. variant according to point 21, adapting favourably to ultra-violet, in which the cathode is made in accordance with point 27, and in which the said solution is made in accordance with point 29;
41. variant according to point 21, adapting favourably to ultra-violet, in which the cathode is made in accordance with point 28, in which the anode is made in accordance with point 23 above, and in which the said electrochromophore in solution is of type p;
42. variant according to point 21, adapting favourably to ultra-violet, in which the cathode is made in accordance with point 28 and in which the solution is made in accordance with point 31 in which the said electrochromophore in solution is of type p;
43. variant according to point 21, which can be used favourably for reversible data storage, in which the cathode is made in accordance with point 27, and in which the said solution is made in accordance with point 30;
44. variant according to point 21, which can be used favourably for reversible data storage, in which the said solution is made in accordance with point 30 and in which a reflective screen is placed behind the anode.
45. variant according to point 44, in which the reflective screen is made of a microcrystalline semiconductor layer, said semiconductor being in accordance with point a above, deposited in the anode face located inside the system;
46. variant according to point 21, which can be used favourably for reversible data storage, in which the said solution is made in accordance with point 30 and in which the cathode is made of a dense layer of electroactive material, capable to be reversibly reduced, said dense layer being deposited on a layer of conductive plastic or glass;
47. variant according to point 46, which can be used favourably for reversible data storage, in which this dense layer is an electroactive organic polymer or a composite material comprising an electroactive material;
48. variant according to points 43 to 47, which can be used favourably for reversible data storage reacting to visible light, in which the anode is made in accordance with point 22;
49. variant according to points 43 to 47, which can be used favourably for data storage reacting to visible light, in which the anode is made in accordance with point 25;
50. variant according to points 43 to 47, which can be used favourably for data storage reacting to visible light, in which the anode is made in accordance with point 26;
51. variant according to points 43 to 47, which can be used favourably for data storage reacting to ultraviolet, in which the anode is made in accordance with point 23;
52. variant according to point 21, in which the said solvent is an electrochemically inert liquid salt, in accordance with point 18, or another liquid, in accordance with point 19;
53. variant according to point 21, in which one of the electrolytes, electrochemically inactive, is selected in accordance with point 20.
The invention likewise relates to electrochromophoric compounds and sensitising agents, in particular in accordance with the following points:
54. type n electrochromophoric compound according to point 1 formed of one or more viologen groups (4,4xe2x80x2-dialkyl-bipyridinium) linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group;
55. type n electrochromophoric compound according to point 54 further comprising a pyrrole, thiophene, vinyl, alcohol, or amine group;
56. type n electrochromophoric compound according to point 1 formed of one or more diimide of naphthalene-1,4,5,8-tetracarboxylic acid groups linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group;
57. type n electrochromophoric compound according to point 56 further comprising a pyrrole, thiophene, vinyl, alcohol, or amine group;
58. type n electrochromophoric compound according to points 54 to 57 such as those represented in the structures below;
59. type p electrochromophoric compound according to point 14, formed of one or more triarylamine groups linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group;
60. type p electrochromophoric compound according to point 59 further comprising a pyrrole, thiophene, vinyl, alcohol, or amine group;
61. type p electrochromophoric compound according to points 59 and 60 such as those represented by the molecule (8) of FIG. 11;
62. sensitising agent compound to which are linked one or more type p electrochromophoric groups according to point 25 and of which the sensitising agent is a ruthenium complex comprising polypyridine ligands of which at least one possesses one or more phosphonate, carboxylate, salicylate, or catecholate groups and at least one or more triarylamine groups;
63. sensitising agent compound to which are linked one or more type p electrochromophoric groups according to point 62 further comprising a pyrrole, thiophene, vinyl, alcohol, or amine group;
64. sensitising agent compound to which are linked one or more type p electrochromophoric groups according to points 62 and 63 such as those represented by the molecules (9) and (10) of FIG. 12.